The present invention relates generally to electrical capacitors which may be variably set or trimmed, and more particularly to methods of programmably adjusting such capacitors to a desired value of capacitance electronically rather than mechanically. It is anticipated that a primary application of the present invention will be for adjusting or tuning such electronically programmable capacitors either before or during their use within more general electronic circuits.
Many electronic circuits require adjustment of particular parameters, either during initial assembly or later, as changing conditions or deterioration due to age, etc. affect such parameters. Of present interest is capacitance, since setting capacitance is critically important to insure the proper operation of a wide variety of electrical circuits and circuit-based processes which are in wide use. Capacitance may, of course, be set by making an initial choice of or performing replacement with a fixed-value component. This will generally not be dealt with here, but it should be kept in mind that substituting an adjustable capacitor for a fixed-value one may be useful in many situations.
Capacitors which may be variably adjusted are desirable, or even critically necessary, in many electronic circuits. For example, computers, clocks, radios, televisions, garage door openers, and a myriad of other electronic devices all have one or more internal circuits that require correct capacitance value adjustment. Adjusting capacitance to achieve such values can be done at the time of initial assembly, where it is almost always necessary, or it can be done later by readjusting back to the original value or by changing to an entirely new capacitance value, as appropriate.
For purposes of the following discussion, the operation of adjusting a variable capacitance device is collectively termed xe2x80x9ctrimming,xe2x80x9d regardless of the specific device type. Further, the operation of setting a variable capacitance device within a more general circuit is collectively termed xe2x80x9ctuning.xe2x80x9d It should be appreciated that these definitions are broad and widely encompassing ones. For example, while some might consider tuning to be proprietary to operations in particular industry segments, such as the adjustment of radio frequency oscillators and amplifiers, and to therefore not also encompass operations like setting delay circuits or stabilizing digital memories, such restrictive interpretation is not intended and is not appropriate here.
Tuning requires determining what capacitance value will be correct in a circuit, and then providing and trimming a xe2x80x9ctuning capacitorxe2x80x9d in the circuit to that value. Unfortunately, this can be an expensive, time-consuming, and error prone process.
FIG. 1 (background art) is a diagram depicting a general circuit 1 requiring capacitive tuning. A conventional analog tuning device 2 is connected to the general circuit 1 for this purpose. The analog tuning device 2 may be as simple as a standard adjustable capacitor, or it may be a complex assembly used to achieve the net effect of adjusting capacitance.
FIG. 2 (background art) depicts the usual choice made today for the analog tuning device 2, a variable analog capacitor 3. Presently the most commonly used variable analog capacitors 3 are mechanical in nature. For example, one type includes several semicircular plates which are rotated relative to other semicircular plates which are fixed. The amount by which the respective plates overlap then determines the capacitance. If the rotating plates do not overlap the fixed plates at all, the capacitance is nominally zero, and if the plates completely overlap, the maximum capacitance of the device is reached. Adjusting to any capacitance between zero and the maximum is thus possible.
Unfortunately, this type of variable analog capacitor 3 has a number of disadvantages. For example, the mechanical relationships of the plates are much subject to undesirable change by shock and vibration. Other, more subtle, influences on the net capacitance are variation in temperature, pressure, and humidity. The dielectric, or inter-plate medium, in such capacitors is often air or another gas, and must usually be kept relatively contained and uncontaminated. Liquid or gel filled adjustable capacitors are also possible but are uncommon, due to concerns such as fill leakage past the seals around adjustment mechanism shafts, etc. Entirely solid dielectrics are not possible, since the plates must permit movement. In sum, variable analog capacitors 3 have numerous inherent characteristics that make them unreliable and failure prone.
Another consideration is utility. To adjust or to readjust the variable analog capacitor 3 requires physical access to perform the mechanical adjustment operation, which is typically rotation. In large and complex systems other components and entire other systems may obscure physical access. In small and compact systems such access may also be difficult, and can even subject surrounding components to potential damage. In hazardous locations, such as the ocean""s depths, physical access can be quite difficult, and in remote locations, such as those visited by space probes, physical access can be outright impossible.
Accordingly, what is needed is a new type of trimable capacitor for use in tuning circuits. This new type of capacitor should not employ mechanical trimming, preferably being electronically trimable. It should also be physically durable, employing all solid and robust materials. It is further desirable that this new type of capacitor permit easy capacitance value adjustment and readjustment.
Of paramount importance for new types of electronically trimable capacitors is the ability to xe2x80x9cprogramxe2x80x9d them. Once a suitable trimable device is provided, and examples will be presently discussed, there remains a need to initially adjust this device, and to readjust its capacitance value as desired. In particular, as digital techniques are increasingly used in electronic circuits, being able to digitally trim such capacitors is highly desirable. Accordingly, what is needed is a method of digitally programming an electronically trimable capacitor.
Accordingly, it is an object of the present invention to provide a method of non-mechanically and digitally programming an electronically trimable capacitor.
And another object of the invention is to provide a method of programming an electronically trimable capacitor once it is incorporated into a more general circuit, to tune the more general circuit thus employing it.
Briefly, one preferred embodiment of the present invention is a method for digitally programming a capacitance value for an electronically adjustable trim capacitor. A program signal to is applied to a program terminal of the adjustable trim capacitor and the capacitance value is set with a logic based on the applied program signal.
An advantage of the present invention is that it does not require physical access to the trimable capacitor for the operation of trimming it, or to a more general circuit employing the trimable capacitor, for the operation of tuning that more general circuit. Since only electronic access is required, trimming and tuning are possible even when physical access is difficult or impossible to obtain, such as within very small assemblies like cell phones, or when in distant locations like outer space.
Another advantage of the invention is that it provides a fast method of programming an electronically trimable capacitor, and thus one that is particularly economical.
Another advantage of the invention is that it provides a reliable method for programming an electronically trimable capacitor, and thus one that is further economical due to the elimination of feedback steps or overall system problems which may result due to error.
And another advantage of the invention is that it provides a method of programming a trimable capacitor which ensures that programming will occur only when it is desired. By the use of digital techniques and appropriate signal conditioning, unintentional signals and spurious influences, like electronic noise, can be prevented from inadvertently miss-programming the electronically trimable capacitor.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.